Dry, high purity nitrogen production process and system

ABSTRACT

Feed air to a prepurifier adsorption system/cryogenic air separation system for dry, high purity nitrogen and/or oxygen production is dried in a membrane dryer preferably characterized by a countercurrent flow path. Drying is enhanced by the use of purge gas on the permeate side of the membrane dryer, which adsorption system or cryogenic air separation system product or waste gas, dried feed air or ambient air being used as purge gas.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the cryogenic separation of air. Moreparticularly, it relates to the pretreatment of feed air to cryogenicair separation systems.

2. Description of the Prior Art

Nitrogen and oxygen are desired for many chemical processing, refinery,metal production and other industrial applications. While varioustechniques are known for the production of nitrogen and/or oxygen by airseparation, cryogenic distillation processes and systems are widely usedfor the production of nitrogen and/or oxygen from air, or for theremoval of nitrogen from well gases. In each cryogenic application, highfreezing point contaminants, which would otherwise solidify at the lowtemperatures at which the primary gas separation takes place, must beremoved from the compressed feed gas stream. Such contaminants arecommonly removed by refrigeration/adsorption process combinations wellknown in the art. In air separation operations, this pre cleanup canutilize a reversing heat exchanger and cold end gel trap combination, ora mechanical air chiller/zeolite molecular sieve adsorber combination.In the former type of processing unit, virtually all of the contaminantsare frozen out of the feed air when the feed air when said air isthermally exchanged against the cryogenic waste and product gas streams.Unfortunately, however, the self cleaning of the reversing heatexchanger unit requires a large purge gas flow relative to the air feed.As a result, the air recovery of such cleanup cycles tends to beundesirably limited Reversing heat exchanger units also require largevalves, which must open and close on a cyclic basis, switching the airfeed and waste purge flow passages. The valves are often located withinthe insulated cold box portion of the cryogenic system, makingmaintenance difficult. Furthermore, to act effectively, the heatexchange-gel trap combination must operate at low temperature, and thusrequires a considerable cool down period during plant start-up.

In contrast to reversing heat exchanger and gel trap combinations,mechanical chiller/adsorptive unit combinations, as disclosed inPrentice, U.S. 4,375,367, can supply a clean, dry feed air stream withinminutes of start-up. The mechanical chiller reduces the air temperatureto about 40° F. from the compressor aftercooler temperature of fromabout 80° F. to about 115° F. The air, which is saturated at the highertemperatures, loses the bulk of its water burden through condensation,thus reducing the inlet water concentration to the adsorptive unit. Theadsorption operation is typically carried out using a pair of pressurevessels, one bed being used for adsorbing purposes, while the other isundergoing regeneration. The pressure vessels are filled with anadsorbent material, such as alumina, zeolite molecular sieve or silicagel, which removes the remaining water vapor, carbon dioxide and/orother contaminants from the feed air stream. The adsorbent beds areusually regenerated at near ambient pressure with a contaminant freestream, either a portion of the cryogenic waste or dry air, which may beheated to improve its desorbing capability. The operation of themechanical chiller substantially improves the performance of theadsorber beds by increasing their adsorption capacity, reducing theinlet water concentration, and, consequently, the purge flow and energyrequirements of the operation. The mechanical chiller is limited to aminimum product dewpoint of about 38° F. due to the necessity foravoiding the buildup of ice on the tubing walls. The chillers must alsobe followed by a moisture separator to remove the condensate formed fromthe feed air and to protect the adsorbent beds from excessive moisture.The mechanical chillers used in such operations tend to be expensive interms of capital and power requirements, especially for small plants. Inaddition, such chillers are generally known for requiring expensivemaintenance.

In light of such factors, there has been a desire in the art for newprocesses and systems that would either eliminate or modify the functionof the components referred to above, particularly the mechanical chillerand moisture separator so as to more economically provide clean, dry airto a cryogenic gas separation unit. One approach considered withinterest is the use of membrane systems to selectively permeate waterfrom feed air. Certain materials are well known as being capable ofselectively permeating water, while air or other gases, comprising lesspermeable components, are recovered as non-permeate gas. A membranesystem utilizing such a material would replace the function of themechanical chiller. Such membrane systems are well known to berelatively simple and easy to operate and maintain. As such membranesystems are normally operated, however, the removal of moisture from thefeed stream requires the co-permeation of significant amounts ofvaluable product gas. Operation of membrane systems at stage cuts on theorder of 10 to 20% might be required to achieve the dewpoint levelachieved by the use of a mechanical chiller. Such circumstance would, asa result, reduce the overall process recovery level achievable, increasethe power requirements of the process, and be generally unattractivefrom an economic viewpoint. Despite such factors serving to deter theuse of membrane dryer systems in place of mechanical chillers or saidreversing heat exchanger and gel trap combinations, the use of membranedryer systems in new, improved overall processes and systems,eliminating the need for the presently employed techniques, wouldrepresent a desirable advance in the art.

It is an object of the invention, therefore, to provide an improvedprocess and system for the production of dry nitrogen and/or oxygenproduct.

It is another object of the invention to provide an improved process andsystem utilizing cryogenic systems for gas separation and providing fordesired for the use of a membrane system for the removal of moisturefrom the feed gas.

It is a further object of the invention to provide a membrane dryersystem capable of achieving enhanced drying efficiency in an overallprocess and system for the recovery of dry nitrogen and/or oxygen usinga cryogenic system for air separation.

With those and other objects in mind, the invention is hereinafterdescribed in detail, the novel features thereof being particularlypointed out in the appended claims.

SUMMARY OF THE INVENTION

A membrane dryer system is employed in conjunction with an adsorptionunit-cryogenic gas separation unit system to achieve a desiredproduction of dry nitrogen and/or oxygen product. The membrane dryer ispreferably operated with a countercurrent flow pattern and is refluxedon the low pressure permeate side thereof. Waste gas from the cryogenicunit is used as purge gas. The area requirements of the membrane arethereby reduced, and the desired product recovery is appreciablyincreased.

BRIEF DESCRIPTION OF THE DRAWING

The invention is hereinafter described in detail with reference to theaccompanying drawings in which:

FIG. 1 is a schematic flow diagram of an embodiment of the invention inwhich the waste gas from the cryogenic feed gas separation system isemployed as purge gas for a membrane system for the drying of the feedgas to the cryogenic system; and

FIG. 2 is a schematic flow diagram of an embodiment in which purge gasremoved from the absorbent bed prepurifier for the cryogenic system isemployed as purge gas for a feed gas membrane drying system.

DETAILED DESCRIPTION OF THE INVENTION

The objects of the invention are accomplished by the integration of amembrane system for feed air drying with a downstreamadsorption-cryogenic air separation system under conditions enablingdesired moisture removal from the feed air to be accomplished withoutreduction in the overall product recovery of the process and system tounacceptable levels. Such conditions advantageously relate to theintegration of the separate processing systems, the selectivity formoisture removal of the particular membrane composition employed, andmembrane bundle design conditions under which countercurrent flow isdesirably achieved in the membrane dryer system. This enables nitrogenand/or oxygen to be recovered in dry form with minimum loss of saidproduct during the drying operation.

In the practice of the invention, waste gas from the cryogenic airseparation system is used to provide purge gas to a membrane dryersystem and to the adsorption system upstream of said cryogenic system.The invention enables a dry, high purity nitrogen and/or oxygen productstream to be obtained with minimum loss of desired product because ofthe requirements of the drying operation. The overall process and systemof the invention is illustrated with reference to the drawings. Furtherinformation relating to the overall cryogenic systems used in thepractice of the invention, and the membrane systems integrated therewithto achieve enhanced drying of feed air are provided below.

In FIG. 1 of the drawings, feed air is passed in line 1 to aircompressor 2, from which wet compressed air is passed in line 3 tomembrane dryer system 4. In said membrane system 4, water selectivelypermeates through the membrane material and is discharged from thesystem as waste gas through line 5. Feed air is recovered from membranedryer system 4 as dry, non permeate or retentate gas through line 6 forpassage to adsorption system 7, which is used to remove contaminantsfrom the dry feed air prior to the passage of said feed air to thecryogenic air separation system. Adsorption system 7 is shown asincluding two beds of adsorbent material, i.e. beds 8 and 9, one bedgenerally being used for its intended adsorption purposes while theother bed is being regenerated. The dry, purified feed gas is passedfrom said adsorption system 7 in line 10 to cryogenic air separationsystem 11, from which the desired dry, high purity product gas isrecovered through line 12. A dry waste stream from said cryogenic systemis withdrawn through line 13. A portion of this dry waste stream, i.eoxygen or nitrogen, is withdrawn through line 14 for passage throughadsorption system 7, that is through either bed 8 or bed 9, as dryadsorbent purge gas for the bed undergoing regeneration. An adsorbentwaste stream is withdrawn from adsorption system 7 through line 15, saidwaste stream containing the adsorbent purge gas and contaminantsdesorbed from the adsorbent beds during the regeneration thereof. Theremaining portion of the dry waste gas from cryogenic air separationsystem 11 is passed through line 16 for introduction to membrane dryersystem 4 as a dry purge gas on the lower pressure, permeate side of saidmembrane system. Said dry purge gas is used to facilitate the removal ofpermeate waste gas from the surface of the membrane, and is discharged,together with said permeate gas, through line 5.

The embodiment of the invention illustrated in FIG. 1 serves toeliminate the need for a chiller otherwise employed as part of achiller/adsorbent bed combination for the removal of water and carbondioxide from the compressed air streams of conventional pre purifiedcryogenic air separation plants. Such elimination of the chiller isdesirable, as indicated above, because it is expensive in terms of bothcapital and power and because it is well known for requiring extensivemaintenance. The membrane dryer system used in the practice of theinvention, on the other hand, is well known as being very simple andinexpensive in nature, and not requiring extensive maintenance. Whilethis embodiment of the invention, integrating membrane systems withadsorption cryogenic air separation systems, is an advantageous advanceover conventional pre purified cryogenic air separation systems, furtherdevelopment in the art is also desirable. One limitation of the FIG. 1embodiment of the invention is that the permeate purge gas requirementsfor the membrane dryer system, which typically are approximately 10-20%of the feed air to said membrane dryer system, are in addition to the10-15% purge requirements for the pre-purifier adsorption system.Consequently, the relatively large overall purge requirements of thesystem, approximately 20-35%, make it difficult to achieve high recoveryof nitrogen and oxygen in cryogenic air separation systems when suchlarge amounts of waste gas are not available.

The embodiment illustrated in FIG. 2 addresses the need for minimizingthe overall purge requirements of the system. In this embodiment, air inline 20 is compressed in air compressor 21, with the compressed airbeing passed in line 22 to coalescer unit 23, from which water isremoved through line 24. The thus-treated compressed air stream ispassed in line 25 to first stage membrane dryer 26, the first part of atwo stage membrane dryer system. Most of the water still present in thefeed air is removed in this first stage dryer, which is refluxed in thepermeate side by a dry purge stream as hereinafter indicated. Thepartially dry, compressed feed air passes, as non-permeate gas, fromfirst stage membrane dryer 26 through line 27 to second stage membranedryer 28, wherein residual water is removed so that dry feed air ispassed therefrom as a non-permeate stream for passage in line 29 toprepurifier adsorption system 30 for purification before passage to thecryogenic air separation system. Adsorption system 30 is shown ascontaining two adsorbent beds, namely bed 31 and bed 32, it beingunderstood that one such bed will commonly be used for purification ofdry feed gas while the other bed is undergoing regeneration. Dry,purified feed air leaving adsorption system 30 is passed in line 33 tocryogenic air separation system 34, from which the desired dry, highpurity nitrogen or oxygen product is recovered through line 35. Drywaste gas from cryogenic system 34 is withdrawn through line 36, heatedin heat exchanger 37, and passed through line 38 to prepurifieradsorption system 30 as purge gas for use in the regeneration ofwhichever bed, i.e. bed 31 or bed 32, is being regenerated at any giventime. Since virtually all of the water present in the feed air isremoved in the membrane dryer system, the spent purge gas exitingprepurifier adsorption system 30 will be relatively dry, although itwill contain other contaminants such as carbon dioxide and hydrocarbons.Such spent purge gas is passed in line 39 to first stage membrane dryer26 for use therein as purge gas on the permeate side of the membrane.Said purge gas, together with water vapor that permeates through saidmembrane dryer 26, is withdrawn through line 40 for discharge to waste.The passage of such recycle purge gas through membrane dryer 26facilitates the carrying of said permeate water away from the surface ofthe membrane on said permeate side so that a high driving force ismaintained across membrane dryer 26 to sustain the desired moistureremoval from the feed air stream being passed to said membrane dryer 26.

Second stage membrane dryer 28 is used, in the FIG. 2 embodiment, to drythe feed air to higher levels than are achieved in first stage membranedryer 26. For purging in this dryer, any dry, low pressure streamavailable from the cryogenic process, such as waste gas from cryogenicsystem 34, high purity nitrogen or oxygen product gas, expanded feed airor the like, or waste gas from prepurifier adsorption system 30, can beused as the dry purge gas. In FIG. 2, a portion of the cryogenic system34 waste gas is shown as being passed through line 41 to second stagemembrane dryer 28 for use as purge gas therein. Such purge gasfacilitates the carrying away of the permeate water from the surface ofthe membrane on the permeate side of the membrane so that a high drivingforce is maintained across membrane 28 to sustain the desired additionaldrying of the feed air stream being passed to said membrane 28. Purgegas, together with additional permeate water, is withdrawn from membranedryer through line 42.

Those skilled in the art will appreciate that the use of second stagemembrane dryer 28 is optional, depending on the degree of feed airdrying desired in any particular dry, high purity nitrogen and/or oxygenproduction operation. When employed, as in the FIG. 2 embodiment, secondstage membrane dryer 28 will typically be smaller and require much lesspurge gas than first stage membrane dryer 26 because most of the waterremoval from the feed air occurs in the first stage membrane dryersystem.

The FIG. 2 embodiment will be seen to be of advantage in that it enablesthe overall purge requirement of the process to be reduced in comparisonto that of the FIG. 1 embodiment. Thus, if the total membrane dryerpurge requirement is 20% and the pre purifier adsorption system 30 purgerequirement is 15%, then, in such embodiment, only 5% of purge gas overand above that employed for pre purification would be required. Removalof virtually all of the water in the membrane dryer also greatly reducesthe water load on the pre purifier adsorption system. This, in turn,greatly reduces the thermal energy required for pre purifierregeneration, making possible perhaps the use of compressor waste heatfor prepurifier regeneration.

Since water is a very strongly adsorbed species in the prepurifier, theremoval of most of the water from the prepurifier feed gas can result inimproved adsorbent performance with respect to other species desired tobe removed, such as carbon dioxide, hydrocarbons and the like. It willbe appreciated that this could lead to desirably improved prepurifieroperation. It should be noted that membrane dryers suitable for theremoval of water will also generally be relatively selective for carbondioxide removal. Such carbon dioxide removal will also reduce the loadon the downstream adsorption unit.

It should also be noted that adsorption of water in the prepurifier isexothermic in nature and generates significant amounts of heat. Thistends to raise the temperature of the air leaving the prepurifier which,in turn, increases the refrigeration load on the cryogenic system.Removal of the water from the prepurifier feed by use of the membranedryer system will tend to greatly reduce the heat generated in theprepurifier adsorption system during adsorption therein, thus benefitingthe downstream cryogenic process.

In the practice of the invention, therefore, it will be seen thatmembrane dryer systems can be effectively integrated with prepurifieradsorption cryogenic air separation systems so as to dry the feed air tosaid adsorption cryogenic systems in a manner representing a highlydesirable advance over the conventional approaches commonly employed inthe art. The membrane dryer system operation is enhanced by the use ofpurge gas on the permeate side of the membrane, with dry waste gas fromthe adsorption cryogenic system, or a portion of the dry, high puritynitrogen product stream from the cryogenic air separation system beingpassed to the membrane dryer system for use therein as said desiredpurge gas.

Certain membranes are known to selectively remove moisture fromcompressed feed air, nitrogen streams or the like. Unfortunately it hasbeen found, as disclosed in U.S. Pat. No. 4,783,201, that, when operatedin a crossflow permeation manner, such membranes may require a stagecut, i.e., the ratio of permeate gas to feed gas flow, of roughly 30%at, for example, 150 psig operation to achieve a relatively modestpressure dewpoint of -40° F. Obviously, the product gas recovery of sucha crossflow membrane unit would be quite low, and the power and dryerarea requirements of such an overall system would be undesirably high.In order to enhance the benefits of the integrated systems in thepractice of the invention, however, the membrane dryer system isdesirably operated in a countercurrent manner, with dry reflux purge gasbeing passed on the permeate side of the membrane to facilitate thecarrying away of moisture from said permeate side and the maintaining ofa high driving force across the membrane for moisture removal. Thisprocessing feature serves to minimize the membrane area required and theproduct permeation loss necessary to achieve a given product dewpoint,i.e. level of drying. It is desirable in preferred embodiments of theinvention, to maintain product loss due to co permeation of saidnitrogen and oxygen from the feed air to less than 1%, preferably lessthan 0.5%, of the total product flow.

It will be appreciated that the membrane composition used in the dryermembrane system should be one having a high selectivity for water overnitrogen and oxygen. That is, moisture must be selectively permeatedmuch more rapidly than air. The water/air separation factor should be atleast 50, preferably greater than 1,000, for advantageous moistureremoval from feed air. As indicated above, such a dryer membrane systemwill also have a carbon dioxide/air separation factor in the range offrom about 10 to about 200. In addition, the membrane composition shouldhave a relatively low permeability rate for both nitrogen and oxygen.Cellulose acetate is an example of a preferred membrane separationmaterial satisfying such criteria. It will be appreciated that a varietyof other materials can also be employed, such as ethyl cellulose,silicone rubber, polyurethane, polyamide, polystyrene and the like.

The membrane dryer system having a membrane material of desirablemembrane composition, which is integrated with a pressure swingadsorption system and cryogenic air separation system as disclosed andclaimed herein, is preferably operated in a countercurrent flow patternas indicated above. In a hollow fiber membrane configuration or in othersuitable membrane configurations, e g. spiral wound membranes, bundledesigns providing for flow patterns of the cross flow type have beencommonly employed in commercial practice. In cross-flow operation, theflow direction of permeate gas on the permeate side of the membrane isat right angles to the flow of feed gas on the feed side of themembrane. For example, in the use of hollow fiber bundles and thepassage of feed gas on the outside of the hollow fiber membranes, theflow direction of permeate in the bores of the fibers is generally atright angles to the flow of feed over the external surface of the hollowfibers. Likewise, in the inside out approach in which the feed gas ispassed through the bores of the hollow fibers, the permeate gasgenerally passes from the surface of the hollow fibers in a directiongenerally at right angles to the direction of the flow of feed withinthe bores of the hollow fibers and then, within the outer shell, in thedirection of the outlet means for the permeate gas. As shown in EuropeanPatent Application Publication No. 0 226 431, published June 24, 1987,countercurrent flow patterns can be created by the encasing of thehollow fiber bundle within an impervious barrier over the entirety ofits longitudinal outer surface except for a non-encased circumferentialregion near one end of said bundle. This enables the feed gas orpermeate gas, depending on the desired manner of operation, i.e.inside-out or outside-in, to pass in countercurrent flow outside thehollow fibers parallel to the flow direction of permeate gas or feed gasin the bores of the hollow fibers. The feed gas on the outside of thehollow fiber bundle, for example, is caused to flow parallel to, ratherthan at right angle to, the central axis of the fiber bundle. It will beunderstood that the membrane fibers may be organized either in straightassemblies parallel to the central axis of the bundle, or alternatively,can be wound in helical fashion around the central axis. In any event,the impermeable barrier material may be a wrap of impervious film, e.g.,polyvinylidene or the like. Alternatively, the impermeable barrier maybe an impervious coating material, e.g. polysiloxane, applied from aninnocuous solvent, or a shrink sleeve installed over the membrane bundleand shrunk onto said bundle. The impermeable barrier thus encases thehollow fiber or other membrane bundle and, as disclosed in saidpublication, has an opening therein permitting the flow of gas into orfrom the bundle so that the fluid flows in a direction substantiallyparallel to the axis of the fiber bundle. For purposes of the invention,the flow pattern should be one of countercurrent flow of the wet feedair stream and the permeate gas comprising purge gas supplied asindicated above, together with moisture that permeates through themembrane material in the membrane dryer system.

It should be noted that membrane drying operations are commonly carriedout in the art using a dense fiber membrane. The membrane thickness forsuch a dense fiber is also the wall thickness, and is very large incomparison to the skin portion of an asymmetric membrane or to theseparation layer of a composite membrane. For a dense fiber, it isnecessary to have a large wall thickness to achieve a significantpressure capability. Thus, dense fibers have a very low permeabilityrate and require the use of a very large surface area for adequatedrying of the nitrogen product. By contrast, asymmetric or compositemembranes, preferred over dense membranes for purposes of the invention,have very thin membrane separation layers, with the relatively moreporous substrate portion of said membranes providing mechanical strengthand support for the very thin portion that determines the separationcharacteristics of the membrane. Much less surface area is required,therefore, for asymmetric or composite membranes than for dense,homogeneous membranes. Because of the inherently improved permeabilityobtainable by the use of asymmetric or composite membranes rather thandense membranes, it is desirable to further enhance asymmetric andcomposite membrane performance in preferred embodiments of theinvention, as related to the drying of feed air, so as to achieve asignificant reduction in the loss of valuable feed air by co-permeationthat would occur in cross-flow operation of such membranes.

It will be understood that the cryogenic air separation system employedfor purposes of the invention can be any conventional, commerciallyavailable system capable of producing high purity nitrogen and/or oxygenin desirable quantities by the cryogenic rectification of air. Thedetails of the cryogenic air separation system are not a part of theessence of the invention, relating to the integration of the cryogenicsystem with a membrane dryer system and with a conventional prepurifieradsorption system. Representative examples of such cryogenic airseparation technology are disclosed in the Cheung patent, U.S.4,448,545, the Pahade et. al. patent, U.S. 4,453,957, and the Cheungpatent, U.S. 4,594,085. Similarly the prepurifier adsorption systememployed in the practice of the invention comprises any desirableadsorption system well known in the art and capable of removingundesired contaminants from the dry feed air stream before its passageto the cryogenic air separation system. The prepurifier adsorptionsystem employed can be any convenient, commercially available systemcapable of removing carbon dioxide and/or other contaminants, includingresidual water, from the dry feed air stream. The adsorption system iscommonly a pressure swing adsorption system operated so as toselectively adsorb said contaminants from the feed air at an elevatedpressure and to desorb said contaminants at lower pressure, e.g. nearambient pressure, for removal from the system. Such pressure systemstypically employ a pair of adsorbent beds, with one bed being used foradsorption purposes while the other bed is being regenerated. Typicaladsorbent materials employed in said beds include alumina, zeolitemolecular sieves or silica gel. Alternately, such systems can beoperated on a thermal swing adsorption cycle, wherein the desiredadsorption is carried but at a lower temperature, with desorption beingaccomplished at an elevated temperature.

For purposes of the invention, a purge ratio, i.e. reflux purge gas/feedair flow on the non permeable side, of at least about 10%, butpreferably about 20% or above, is desired to keep area requirements,product loss and back diffusion to a minimum. The purge ratiorequirements also tend to be greater at relatively lower feed airpressures than at higher pressures.

In an illustrative example of the practice of the invention, thecryogenic air separation system is adapted to produce 50 tons of dry,high purity nitrogen. Since nitrogen recovery based on air in theconventional pre-purified cryogenic system is typically on the order of52%, approximately 48% of the feed air flow is available as low pressurewaste. The cryogenic system can conveniently be operated with a feed airpressure of 91 psia, at an air temperature of 115° F., with a waste gaspressure of 18 psia. In a conventional system, an aftercooler dewpointof 115° F., chiller product air dewpoint of 40° F., and an absorbentproduct air dewpoint of -100° F. can conveniently be employed. Aconventional mechanical chiller for use in such a system would costapproximately $30,000 and consume about 10 KW of electrical power. Theair pressure drop in such a chiller and moisture separator would be onthe order of about 2 psi. The chiller is desirably replaced in thepractice of the invention, as in the FIG. 1 embodiment, with a membranedryer system having an oxygen/nitrogen separation factor of 5.9, and awater/air separation factor of 1,000 or more. The membrane dryer systemis desirably comprised of hollow fiber membranes wound in a helicalconfiguration, and operated using an impervious barrier ofpolyvinylidene to encase the membrane and create a countercurrent flowpattern. In order to minimize the amount of compressed air lost due topermeation during the drying operation, the stage cut, i.e.permeate/feed flow, of the membrane is kept very low. However, it shouldbe recognized, as indicated above, that a portion of the actualoperating stage cut is due to the desired rejection of water and isunavoidable if the desired drying is to be achieved. For enhanceddrying, therefore, it is the dry stage cut resulting from theco-permeation of oxygen and nitrogen that is minimized, i.e. to not morethan about 5%, preferably to less than 0.5% of the inlet feed air. A dryreflux purge ratio on the order of 18-20% is used under the particularoperating conditions and membrane characteristics referred to above. Themembrane dryer system is found to achieve a significant reduction incapital and power costs, and other benefits, provided that said dry fluxpurge ratio of at least 18% is available.

An added advantage of the membrane dryer system is that it is notlimited to providing a 40° F. air dewpoint feed to theadsorption-cryogenic system. A given membrane area can be used toprovide air of varying quality depending on the purge ratio employed andthe membrane characteristics. The residual water concentration of thedried air can be reduced by the use of more purge gas, or membranes withhigher water separation characteristics, apart from the use of increasedmembrane area. Any such reduction in residual water content will serveto reduce the amount of water vapor that must be removed by theadsorbent beds in the prepurifier adsorption system, thereby increasingthe capacity of said system and reducing the purge gas and energyrequirements thereof. The optimum membrane dryer dewpoint will thus beseen to depend on the relative cost of removing water in the membranedryer system and in the prepurifier adsorption system.

It will be appreciated that various changes and modifications can bemade in the details of the process and system as herein describedwithout departing from the scope of the invention as set forth in theappended claims. Thus, asymmetric or composite membrane structures canbe employed in the dryer membrane system of the invention. While densemembranes are commonly used for product drying applications, such densemembranes are not preferred because of the inherent limitations thereofnoted above, although they can be used in the practice of the invention.

The permeable membranes employed in the practice of the invention willcommonly be employed in assemblies of membrane bundles, typicallypositioned within enclosures to form membrane modules that comprise theprincipal element of a membrane system. A membrane system may comprise asingle module or a number of such modules, arranged for either parallelor series operation. The membrane modules can be constructed usingbundles of membranes in convenient hollow fiber form, or in spiralwound, pleated flat sheet, or other desired membrane configurations.Membrane modules are constructed to have a feed air side, and anopposite, permeate gas exit side. For hollow fiber membranes, the feedside can be either the bore side for inside out operation, or theoutside of the hollow fibers for outside in operation. Means areprovided for introducing feed air to the system and for withdrawing bothpermeate and non permeate streams.

As indicated above, the purge gas employed in the invention should be adry or a relatively dry gas, as from the sources referred to herein. Asused herein, a relatively dry purge gas is one having a moisture partialpressure not exceeding the partial pressure of moisture in the driedfeed air stream. Preferably, said purge gas moisture partial pressurewill be less than half the moisture partial pressure in said stream, aswill be the case with respect to the sources of purge gas disclosedabove.

Membranes will be seen to provide a highly desirable system and processfor drying feed air before its passage to air adsorption-cryogenic airseparation system for the production of dry, high purity nitrogen. Byaccomplishing the drying in convenient membrane systems, the use of themore costly chillers for moisture removal can be avoided. By integratingthe processing streams of the membrane dryer system with the cryogenicair separation system and the prepurifier adsorption system, a purge ofthe low pressure, permeate side of the membrane dryer system withrelatively dry purge gas is conveniently accomplished. By utilizing abundle arrangement so as to establish a countercurrent flow pattern,preferred embodiments of the drying operation can be carried out with anenhanced recovery of dry feed air, avoiding the co-permeation ofsignificant amounts of compressed air as occurs in cross-flow permeationoperations.

What is claimed
 1. An improved system for the production of dry, highpurity nitrogen and/or oxygen from air comprising:(a) a membrane dryersystem capable of selectively permeating water present in wet feed air;(b) a prepurification adsorption system capable of selectively adsorbingcarbon dioxide, residual water and other contaminants from dry feed airremoved as non permeate gas from said membrane dryer system; (c) acryogenic air separation system for the cryogenic rectification of air,and the production of dry, high purity nitrogen and/or oxygen productgas, together with a dry waste gas; (d) conduit means for passingrelatively dry purge gas to the low pressure permeate side of themembrane dryer system to facilitate the carrying away of water vaporfrom the surface of the membrane and maintaining the driving force forremoval of water vapor through the membrane from the feed air stream forenhanced moisture separation therefrom, said relatively dry purge gascomprising waste or product gas from said cryogenic air separationsystem and/or the prepurifier adsorption system or ambient air, wherebythe provision of purge gas on the permeate side of the membrane dryersystem facilitates the desired moisture removal with minimum loss offeed air.
 2. The system of claim 1 in which said membrane dryer systemcontains membrane bundles adapted for a countercurrent flow pattern withthe permeate gas flowing generally parallel to the flow of wet feed air.3. The system of claim 1 in which said dry purge gas for the membranedryer system comprises waste gas from said cryogenic air separationsystem.
 4. The system of claim 3 and including conduit means for passinga portion of said waste gas from the cryogenic air separation system tosaid prepurifier adsorption system as purge gas therefor.
 5. The systemof claim 1 and including conduit means for passing waste gas from saidcryogenic air separation system to said prepurifier adsorption system aspurge gas therefor, the waste gas from said prepurifier adsorptionsystem comprising said purge gas for the membrane dryer system.
 6. Thesystem of claim 5 in which said membrane dryer system comprises atwo-stage membrane system, said waste gas from the prepurifieradsorption system being passed to the first stage of said membranesystem as purge gas.
 7. The system of claim 6 and including additionalconduit means to pass waste or product gas from said cryogenic airseparation system or expanded air to the second stage of said membranesystem as purge gas.
 8. The system of claim 5 in which said membranedryer system contains membrane bundles adapted for a countercurrent flowpattern with the permeate gas flowing generally parallel to the flow ofwet feed air.
 9. An improved process for the production of dry, highpurity nitrogen and/or oxygen from air comprising:(a) passing wet feedair to a membrane dryer system capable of selectively permeating watertherefrom; (b) passing the thus dried feed air to a pre-purificationadsorption system capable of selectively adsorbing carbon dioxide,residual water and other contaminants from dry feed air removed asnon-permeate gas from said membrane dryer system; (c) passing the dry,pre-purified feed air from said pre-purification adsorption system to acryogenic air separation system for the cryogenic rectification of air,and the production of dry, high purity nitrogen product gas, togetherwith a dry, oxygen-containing waste gas; (d) recovering dry, high puritynitrogen product gas from said cryogenic air separation system; and (e)passing relatively dry purge gas to the low pressure permeate side ofthe membrane dryer system to facilitate the carrying away of water vaporfrom the surface of the membrane and maintaining the driving force forremoval of water vapor through the membrane from the feed air stream forenhanced moisture separation therefrom, said relatively dry purge gascomprising waste or product gas from said cryogenic air separationsystem and/or the prepurifier adsorption system or ambient air, wherebythe provision of purge gas on the permeate side of the membrane dryersystem facilitates the desired moisture removal with minimum loss offeed air.
 10. The process of claim 9 in which said membrane dryer systemcontains membrane bundles adapted for a countercurrent flow pattern withthe permeate gas flowing generally parallel to the flow of wet feed air.11. The process of claim 9 in which said dry purge gas for the membranedryer system comprises waste gas from said cryogenic air separationsystem.
 12. The process of claim 11 and including passing a portion ofsaid waste gas from the cryogenic air separation system to saidprepurifier adsorption system as purge gas therefor.
 13. The process ofclaim 9 and including passing waste gas from said cryogenic airseparation system to said prepurifier adsorption system as purge gastherefor, the waste gas from said pre-purifier adsorption systemcomprising said purge gas for the membrane dryer system.
 14. The processof claim 13 in which said membrane dryer system comprises a two-stagemembrane system, and in which waste gas from the prepurifier adsorptionsystem is passed to the first stage of said membrane system as purgegas.
 15. The process of claim 14 and including passing waste or productgas from said cryogenic air separation system or expanded air to thesecond stage of said membrane system as purge gas.
 16. The process ofclaim 15 in which waste gas from the cryogenic air separation system ispassed to the second stage of said membrane system as purge gas.
 17. Theprocess of claim 13 in which said membrane dryer system containsmembrane bundles adapted for a countercurrent flow pattern with thepermeate gas flowing generally parallel to the flow of wet feed air.